The M40 recoilless rifle is a lightweight,[3] portable, crew-served 105 mm weapon intended primarily as an anti-tankweapon made in the United States. The weapon is commonly described as being 106 mm, but it is in fact 105 mm; the 106 mm designation was designed to prevent confusion with the incompatible 105 mm ammunition from the failed M27.[4] It could also be employed in an antipersonnel role with the use of the antipersonnel-tracer flechette round. It can be fired primarily from a wheeled ground mount. The air-cooled, breech-loaded, single-shotrifle fired fixed ammunition. It was designed for direct firing only, and sighting equipment for this purpose was furnished with each weapon.

The M27 recoilless rifle was a 105-mm weapon developed in the early 1950s and fielded in the Korean War. Although a recoilless rifle of this caliber had been a concept since the Second World War, the weapon was hurriedly produced with the onset of the Korean War. The speed with which it was developed and fielded resulted in problems with reliability caused by trunnions that were mounted too far to the rear. The M27 was also considered too heavy by the U.S. Army and had a disappointing effective range due to the lack of a spotting rifle. Taking the M27 as a the basis for a new design, the Army developed an improved version of the M27 that was in 1955 type-designated the M40 106-mm recoilless rifle.[5] Originally along with its type-designation it was also given the official name BAT for Battalion Anti-Tank gun, but that was soon dropped.[6] Although unsuitable for military purposes, M27 recoilless rifles were used to trigger controlled avalanches at ski resorts and mountain passes in the United States.[7]

The M40 primarily saw action during the Vietnam War and was later replaced by the BGM-71 TOW anti-tank missile system. The weapon was also used by anti-communist forces in Angola mounted on Land Rovers.

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The M40 is shaped like a long tube with an M8 0.50 cal spotting rifle above. The spotting rifle fires a round whose trajectory closely matches that of the 106 mm round and gives off a puff of smoke on impact with the target. On the left hand side, there is an elevating wheel, in the centre of which is the trigger wheel used to fine adjust the elevation and at the same time firing the spotting rifle when pulled, and the gun when pushed. The mounting is a tripod, but the front leg has a castoring wheel. On top of the mount is a traverse wheel. On the centre of the traverse wheel is a locking wheel, when the wheel is down, the rifle is locked in traverse, and can only be moved right and left with the traverse wheel. When the wheel is raised, the rifle can be traversed by hand. Austria produced a two-wheeled mount for the M40.

A special vehicle called Ontos carried six M40s. A version specific to the T195E5 mount, the M40A1C, was used. It was used only by the U.S. Marine Corps. Japan produced a self-propelled gun called the Type 60 which carried two side by side. Some Pakistani M113s have a dual mounting.

Ammunition for the 105 mm rifle was issued as one-piece fixed cartridges. The term "fixed" means that the projectile and the cartridge case are crimped together. This ensures correct alignment of the projectile and the cartridge case. It also permits faster loading because the projectile and the cartridge case are loaded as one unit. The rear end of the cartridge case is perforated, to allow the propellant gas to escape through the vented breech, thus neutralizing recoil. Most projectiles (except HEAT) used are pre-engraved, that is, the rotating bands are cut to engage the rifled bore.[10] If the round was not rotated slightly when loading the M40 it could result in jamming in the breach.

Types of ammunition included HEAT, High Explosive Plastic-Tracer (HEP-T), canister, High Explosive Anti Personnel, and the M368 dummy round which could not be fired and was used for crew drill. The original U.S. HEAT round penetrated more than 400 mm of armor. Near the end of the M40's service life, both Austria and Sweden produced HEAT rounds for the weapon capable of penetrating more than 700 mm of armor.[11]

The ammunition for the 0.50 cal spotting rifle is not .50 BMG. The round used is a special round designed to simulate the flight path of the 105 mm ammunition.

Although the spotting rifle could conceivably be used in an antipersonnel role, historical U.S. military doctrine strongly discouraged this use. This limitation was entirely tactical in nature, having been intended to help conceal the vulnerable M40 and its crew from the enemy before the main anti-tank gun could be fired; however, this restriction is believed to be the source of a long-standing misconception that the laws of war restrict the use of .50-caliber projectiles against enemy personnel more generally.[15]